The oscillation suffered by the Earth’s axis due to the precession of its orbit changes climatic conditions
Not always the Sahara it was a desert. A group of scientists has shown that over time this region has had periodic phases of ‘greening’ and increased humidity, which repeat every 21,000 years and they are a consequence of the Earth’s orbit. In this way, the Sahara has cyclically been forest, savannah and desert.
The research, published in Nature Communicationsdemonstrates that the humid and green phases that have periodically occurred in the Sahara were driven by changes in the Earth’s orbit around the Sun and, on the other hand, they were suppressed during the ice ages.
For the first time, climate scientists have simulated the historical “greening” intervals of the Sahara, obtaining evidence of how the duration and intensity of these wet events They were also influenced by the effects of ice periods in the Northern Hemisphere.
Lead author Edward Armstrong, a climate scientist at the University of Helsinki and the University of Bristol, said: “The cyclical transformation of the Sahara Desert into savannah and forest ecosystems It is one of the most notable environmental changes on the planet”.
“Our study is one of the first climate modeling studies to simulate African wet periods with a magnitude comparable to what paleoclimatic observations indicate, revealing why and when these events occurred,” he added.
A Sahara with rivers and lakes
There is widespread evidence that The Sahara was periodically covered with vegetation in the past, with the proliferation of rivers, lakes and water-dependent animalslike hippos, before becoming what is now a desert.
These wet periods in North Africa may have been crucial in providing vegetated corridors to other continents, allowing the dispersal of several species, including early humans, around the world.
Through the Earth’s orbit
The so-called ‘greenings’ are believed to have been driven by changes in Earth’s orbital conditions, specifically Earth’s orbital precession. Precession is a movement by which the Earth oscillates on its axis, which influences the seasons during a cycle of approximately 21,000 years. These precession changes determine the amount of energy the Earth receives in different seasons, which in turn controls the strength of the African monsoon and the expansion of vegetation in this vast region.
A major obstacle to understanding these events is that most climate models have not been able to simulate the extent of these wet periods, so the specific mechanisms driving them remain uncertain.
This study, however, used a recently developed climate model to simulate the wet periods of North Africa and thus better understand the mechanisms that drove them.
The results confirm that North Africa’s wet periods occurred every 21,000 years due to changes in the Earth’s orbital precession. This caused warmer summers in the northern hemisphere, which intensified the strength of the West African monsoon system and increased Saharan rainfallwhich caused the spread of savanna-type vegetation throughout the desert.
Disruption during ice ages
The findings also show that These wet periods did not occur during the ice ages, when there were large sheets of glacial ice covering much of the high latitudes. This is because these vast ice sheets cooled the atmosphere and suppressed the tendency of the African monsoon system to expand.
This highlights an important interconnection between these distant regionswhich may have restricted the dispersal of species, including humans, out of Africa during glacial periods of the last 800,000 years.
Co-author Paul Valdés, Professor of Physical Geography at the University of Bristol, said: “We are very excited about the results. Traditionally, climate models have struggled to represent the extent of the Sahara’s ‘greening’. Our revised model successfully represents past changes and also gives us confidence in your ability to understand future changes”.
Co-author Miikka Tallavaara, assistant professor of hominin environments at the University of Helsinki, said: “The Sahara region is a kind of gateway that controls the dispersal of species between northern and sub-Saharan Africa, and on and off the continent,” .
“The door was open when the Sahara was green and closed when deserts prevailed. This alternation of humid and arid phases had important consequences for the dispersal and evolution of species in Africa. “Our ability to model wet periods in North Africa is an important achievement and means that we can now also better model human distributions and understand the evolution of our genus in Africa.”
Reference study: https://www.nature.com/articles/s41467-023-41219-4
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